1. Field of the Invention
The present invention relates to formation of a silicon oxide film used as an insulation film in a semiconductor device or a liquid crystal device, particularly, to a method and apparatus for depositing a silicon oxide film from a supersaturated hydrofluoric acid solution of silicon oxide at low temperatures.
2. Description of the Related Art
A silicon oxide (SiO.sub.2) film, which is excellent in its mechanical strength and insulating properties, is used in various fields. Particularly, a silicon oxide film is used in a semiconductor device as an interlayer insulating film, a capacitor oxide film, an impurity diffusion source, a gate oxide film, a protective film, etc. Also, where an alkali-containing glass such as a soda-lime glass or a borosilicate glass is used as a substrate glass in a liquid crystal display panel or a solar cell, the glass surface is covered with a silicon oxide film for preventing elution of the alkali component. Further, a silicon oxide film is used as a protective film for a plastic substrate surface of an optical disk.
Known methods for forming a silicon oxide film include, for example, vacuum vapor deposition, sputtering, CVD and thermal oxidation of silicon. Recently, a method utilizing precipitation from a supersaturated hydrofluoric acid solution of silicon oxide is employed for forming a silicon oxide film, as disclosed in, for example, Published Examined Japanese Patent Application No. 1-27574 and Japanese Patent Application No. 2-418924. The silicon oxide film formed by the precipitation method exhibits a high covering property and a high insulation breakdown voltage. Also, the silicon oxide film can be formed at low temperatures. It follows that the precipitation method is useful when employed for the manufacture of, particularly, a semiconductor device. The silicon oxide film formed by the precipitation method is generally called an SORD (Silicon Oxide Room Temperature Deposition) film, a SORD SiO.sub.2 film, or an LPD (Liquid Phase Deposition) film.
The precipitation method for forming a silicon oxide film on a substrate is particularly useful in the manufacture of a semiconductor device. In forming a silicon oxide film by the precipitation method, a saturated solution of silicon oxide is prepared by adding silicon oxide (silica) to hydrofluoric acid until the solution is saturated. In this case, silicon oxide reacts with hydrofluoric acid (HF) to form a saturated state as given below: EQU H.sub.2 SiF.sub.6 +2H.sub.2 O.revreaction.SiO.sub.2 +6HF (1)
Then, aluminum is added to the saturated hydrofluoric acid solution of silicon oxide. As a result, aluminum reacts with hydrofluoric acid to form aluminum fluoride and hydrogen. If hydrofluoric acid is consumed by its reaction with aluminum, silicon oxide becomes excessive in the solution so as to form a supersaturated hydrofluoric acid solution of silicon oxide. Silicon oxide in the supersaturated solution is deposited on the surface of a substrate such as a semiconductor wafer dipped in the solution.
In the precipitation method outlined above, it is possible to use, for example, iron or boric acid in place of aluminum for promoting the silicon oxide precipitation. Also, the silicon oxide solubility in hydrofluoric acid is increased with temperature drop. Thus, if a solution saturated with silicon oxide at low temperatures is left to stand at high temperatures, the solution becomes supersaturated, leading to precipitation of silicon oxide.
FIG. 17 shows an apparatus for forming a silicon oxide film by the conventional precipitation method described above. As shown in the drawing, a film-forming vessel 1 is filled with a supersaturated hydrofluoric acid solution of silicon oxide 3. The supersaturated solution 3 overflowing from the vessel 1 is received by an over-flow vessel 2 disposed in communication with the vessel 1 and, then, flows through a pipe 4 connected to the bottom of the over-flow vessel 2 back into the film-forming vessel 1. It is seen that the supersaturated solution 3 flowing through the pipe 4 passes through a pump 5 and a filter 6, with the result that the larger precipitated particles and dust entering the solution during the reaction or the circulation are removed from the supersaturated solution 3.
An aluminum plate 7 is dipped in the solution 3 so as to make the solution 3 within the film-forming vessel 1 supersaturated with silicon oxide. Specifically, aluminum within the over-flow vessel 2 is dissolved in the solution and reacts with hydrofluoric acid, as described previously, with the result that the equilibrium of the film-forming solution denoted by formula (1) is collapsed so as to permit precipitation of silicon oxide on the surface of, for example, a semiconductor wafer 8 dipped in the film-forming solution 3.
The precipitation method described above does not necessitate a vacuum apparatus and a high temperature reaction apparatus and, thus, is superior to CVD method in the cost of an apparatus forming a silicon oxide film. Also, even if the substrate has a stepped portion, the substrate can be covered with a silicon oxide film of a uniform thickness. In other words, the silicon oxide film formed by the precipitation method is superior to that formed by CVD in the step coverage property.
In the conventional precipitation method, however, the film-forming rate is as low as about 1000 .ANG./H (hour). In other words, it takes as much as 10 hours to form a silicon oxide film having a thickness of, for example, 1 .mu.m. It should also be noted that precipitation takes place everywhere within the film-forming solution 3, with the result that as small as only about 1% of silicon oxide precipitated from the solution is deposited on the substrate 8. The major portion of the precipitated silicon oxide is caught by the filter 6, deposited on the inner surface of the film-forming vessel, or discarded in the step of washing the filter. It follows that the utilization rate of the raw material silicon oxide is very low in the conventional precipitation method. What should also be noted is that the silicon oxide deposited on the inner surface of the film-forming vessel is peeled to form a lump, which is attached as dust to the substrate 8. Naturally, bad effects are given by the attached dust in the subsequent steps of manufacturing a semiconductor device.
It is certainly possible to improve the film-forming rate in the conventional precipitation method by elevating the temperature of the film-forming solution 3 which is kept in general at 35.degree. C. or by increasing the amount of the aluminum plate 7 so as to increase the silicon oxide precipitation rate. In this case, however, the amount of the precipitated silicon oxide particles caught by the filter 6 is also increased, with the result that the filter 6 is plugged in a short time and, thus, the silicon oxide film formation on the substrate 8 is impaired. The highest film-forming rate achieved by the conventional precipitation method, in which the precipitation reaction is promoted without impairing the oxide film formation, is as low as only about 1400 .ANG./H. Even in this case, the utilization rate of the precipitated silicon oxide particles remains very low. In addition, a silicon oxide film is likely to be formed easily on the inner surface of the film-forming vessel in accordance with increase in the film-forming rate. The silicon oxide film thus formed tends to peel off and to be attached to the substrate 8, with the result that the subsequent steps for forming fine patterns are seriously impaired.
Further, it may be possible to enlarge the film-forming vessel 1 so as to permit a simultaneous processing of a large number of substrates. This measure certainly permits increasing the through-put of the film-forming apparatus. However, where a very large number of steps are involved in the manufacturing process of the desired article as in the manufacture of an LSI, it is desirable to diminish the processing time for each step as much as possible in order to shorten the time required for the manufacture of the desired article.